Compositions which crosslink to give elastomers and which are intended to have low compression set are known in the art.
DE 41 00 217 (Degussa AG) discloses that thiocyanatopropyltrialkoxysilanes can bring about an improvement in the compression set of molding compositions which can undergo peroxidic vulcanization when exposed to heat. It is said here that the organosilicon compounds can be used in isolation or after being converted using a filler. It is likewise known from DE 33 19 251 (Degussa AG) that isothiuronium compounds may be used instead of thiourea derivatives for vulcanizable halogen rubber mixtures. The resultant vulcanizates have better physical properties than comparable halogen rubbers without isothiuronium compounds in strength, modulus, abrasion and compression set.
In EP 0 278 157 (Shin Etsu Chemical Co., Ltd.) thiocarbamyl-functional organosilanes have been added to improve compression set to rubbers which can be vulcanized with organic sulfur compounds or with organic peroxides.
Other proposals for improving compression set are based on the use of additives, such as barium peroxide, barium oxide or magnesium oxide (U.S. Pat. No. 3,261,801, U.S. Pat. No. 3,468,838 and U.S. Pat. No. 3,865,778) and on the use of polycarbodiimide-polysiloxane copolymers (EP 0 010 708), which give strongly colored products.
The above-mentioned processes for improving compression set relate to rubbers which can be crosslinked using organic sulfur compounds or using organic peroxides, to give elastomers.
In contrast, organopolysiloxane compositions are crosslinked by an addition reaction, preferably a hydrosilylation reaction of vinyl-containing siloxanes catalyzed by hydrosilylation catalysts, including a wide variety of platinum compounds. Compounds hitherto described for improving compression set in addition-crosslinking systems are iron-manganese spinels of the empirical formula Fe.sub.y Mn.sub.z O.sub.4 (Wacker Silicones: EP 582 299), triazole compounds (Shin Etsu Chemical Co., Ltd.: U.S. Pat. No. 5,104,919), vinyl-specific organic peroxides &gt;0.2% by weight (Dow Corning Co.: EP 0 377 185) and transition metal chalcogenides (Bayer AG: EP 0 455 078). The above-mentioned compositions have the disadvantage of still not providing addition-crosslinking compositions with low compression set values.
In EP 0 691 365 (Ernst Sonderhoff GmbH & Co), the compression set of foams is improved by increasing the Pt content and annealing for from 15 to 20 minutes at from 120 to 150.degree. C. This process has the disadvantage of requiring an additional heat treatment, and also an increased amount of Pt as catalyst.